Download hw02.zip. Inside the archive, you will find a file called hw02.py, along with a copy of the ok autograder.
Submission: When you are done, submit with python3 ok --submit. You may submit more than once before the deadline; only the final submission will be scored. Check that you have successfully submitted your code on okpy.org. See Lab 0 for more instructions on submitting assignments.
Using Ok: If you have any questions about using Ok, please refer to this guide.
Readings: You might find the following references useful:
Grading: Homework is graded based on correctness. Each incorrect problem will decrease the total score by one point. There is a homework recovery policy as stated in the syllabus. This homework is out of 2 points.
Several doctests refer to these functions:
from operator import add, mul
square = lambda x: x * x
identity = lambda x: x
triple = lambda x: 3 * x
increment = lambda x: x + 1
These videos may provide some helpful direction for tackling the coding problems on this assignment.
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To work on these problems, open the Parsons editor:
python3 parsons
Implement the function count_until_larger. count_until_larger takes in a positive integer num. count_until_larger counts the distance between the rightmost digit of num and the nearest greater digit; to do so, the function counts digits from right to left. Once it encounters a digit larger than the rightmost digit, it returns that count. If no such digit exists, then the function returns -1.
For example, 8117 has a rightmost digit of 7 and returns a count of 3. 9118117 also returns a count of 3: for both, the count stops at 8.
0 should be treated as having no digits and returns a count of -1.
Consult the following doctests for specific behaviors of count_until_larger.
def count_until_larger(num):
"""
Complete the function count_until_larger that takes in a positive integer num.
count_until_larger examines the rightmost digit and counts digits from right to
left until it encounters a digit larger than the rightmost digit, then returns that count.
>>> count_until_larger(117) # .Case 1
-1
>>> count_until_larger(8117) # .Case 2
3
>>> count_until_larger(9118117) # .Case 3
3
>>> count_until_larger(8777) # .Case 4
3
>>> count_until_larger(22) # .Case 5
-1
>>> count_until_larger(0) # .Case 6
-1
"""
"*** YOUR CODE HERE ***"
Write a function filter_sequence which takes in two integers, start and stop, as well as a function cond, which takes in a single argument and outputs a boolean value. filter_sequence returns the sum of all digits from start to stop (inclusive) for which cond returns True.
def filter_sequence(cond, start, stop):
"""
Returns the sum of numbers from start (inclusive) to stop (inclusive) that satisfy
the one-argument function cond.
>>> filter_sequence(lambda x: x % 2 == 0, 0, 10) # .Case 1
30
>>> filter_sequence(lambda x: x % 2 == 1, 0, 10) # .Case 2
25
"""
"*** YOUR CODE HERE ***"
Douglas Hofstadter’s Pulitzer-prize-winning book, Gödel, Escher, Bach, poses the following mathematical puzzle.
n as the start.n is even, divide it by 2.n is odd, multiply it by 3 and add 1.n is 1.The number n will travel up and down but eventually end at 1 (at least for all numbers that have ever been tried – nobody has ever proved that the sequence will terminate). Analogously, a hailstone travels up and down in the atmosphere before eventually landing on earth.
This sequence of values of n is often called a Hailstone sequence. Write a function that takes a single argument with formal parameter name n, prints out the hailstone sequence starting at n, and returns the number of steps in the sequence:
def hailstone(n):
"""Print the hailstone sequence starting at n and return its
length.
>>> a = hailstone(10)
10
5
16
8
4
2
1
>>> a
7
>>> b = hailstone(1)
1
>>> b
1
"""
"*** YOUR CODE HERE ***"
Hailstone sequences can get quite long! Try 27. What’s the longest you can find?
Note that if
n == 1initially, then the sequence is one step long.
Use Ok to test your code:
python3 ok -q hailstone
Curious about hailstones or hailstone sequences? Take a look at these articles:
The summation(n, term) function from the higher-order functions lecture adds up term(1) + ... + term(n). Write a similar function called product that returns term(1) * ... * term(n).
def product(n, term):
"""Return the product of the first n terms in a sequence.
n: a positive integer
term: a function that takes one argument to produce the term
>>> product(3, identity) # 1 * 2 * 3
6
>>> product(5, identity) # 1 * 2 * 3 * 4 * 5
120
>>> product(3, square) # 1^2 * 2^2 * 3^2
36
>>> product(5, square) # 1^2 * 2^2 * 3^2 * 4^2 * 5^2
14400
>>> product(3, increment) # (1+1) * (2+1) * (3+1)
24
>>> product(3, triple) # 1*3 * 2*3 * 3*3
162
"""
"*** YOUR CODE HERE ***"
Use Ok to test your code:
python3 ok -q product
Let’s take a look at how summation and product are instances of a more general function called accumulate, which we would like to implement:
def accumulate(merger, start, n, term):
"""Return the result of merging the first n terms in a sequence and start.
The terms to be merged are term(1), term(2), ..., term(n). merger is a
two-argument commutative function.
>>> accumulate(add, 0, 5, identity) # 0 + 1 + 2 + 3 + 4 + 5
15
>>> accumulate(add, 11, 5, identity) # 11 + 1 + 2 + 3 + 4 + 5
26
>>> accumulate(add, 11, 0, identity) # 11
11
>>> accumulate(add, 11, 3, square) # 11 + 1^2 + 2^2 + 3^2
25
>>> accumulate(mul, 2, 3, square) # 2 * 1^2 * 2^2 * 3^2
72
>>> # 2 + (1^2 + 1) + (2^2 + 1) + (3^2 + 1)
>>> accumulate(lambda x, y: x + y + 1, 2, 3, square)
19
>>> # ((2 * 1^2 * 2) * 2^2 * 2) * 3^2 * 2
>>> accumulate(lambda x, y: 2 * x * y, 2, 3, square)
576
>>> accumulate(lambda x, y: (x + y) % 17, 19, 20, square)
16
"""
"*** YOUR CODE HERE ***"
accumulate has the following parameters:
term and n: the same parameters as in summation and productmerger: a two-argument function that specifies how the current term is merged with the previously accumulated terms.start: value at which to start the accumulation.For example, the result of accumulate(add, 11, 3, square) is
11 + square(1) + square(2) + square(3) = 25
Note: You may assume that
mergeris commutative. That is,merger(a, b) == merger(b, a)for alla,b, andc. However, you may not assumemergeris chosen from a fixed function set and hard-code the solution.
After implementing accumulate, show how summation and product can both be defined as function calls to accumulate.
Important: You should have a single line of code (which should be a return statement) in each of your implementations for summation_using_accumulate and product_using_accumulate, which the syntax check will check for.
Use Ok to test your code:
python3 ok -q accumulate
python3 ok -q summation_using_accumulate
python3 ok -q product_using_accumulate
Make sure to submit this assignment by running:
python3 ok --submit
Homework assignments will also contain prior exam-level questions for you to take a look at. These questions have no submission component; feel free to attempt them if you’d like a challenge!
Note that exams from Spring 2020, Fall 2020, and Spring 2021 gave students access to an interpreter, so the question format may be different than other years. Regardless, the questions included remain good exam-level problems doable without access to an interpreter.